1. Field of the Invention
The present invention relates to a focus search method of performing a focus search for matching a position of each recording layer of a digital disk such as a DVD with a focal position of an optical beam, and a controller for implementing the method. The digital disk is used for recording and reproducing information by employing an optical beam, and has a plurality of recording layers.
2. Description of the Related Art
Hitherto, in the case of a digital disk for recording and reproducing information by using an optical beam, pits are formed in a surface of the digital disk to record information. To reproduce the recorded information from the digital disk, focus control for focusing an optical beam on a recording layer is carried out. In order to expand storage capacity, digital disks such as DVDs have come to be used. As shown in FIG. 11, this type of digital disk is equipped with a plurality of recording layers produced by forming a second recording layer 62 on a substrate 61, forming a transparent intermediate layer 63 on a surface of the second recording layer 62, forming a first recording layer 64 on a surface of the intermediate layer 63, and further forming a transparent protective layer 65 on a surface of the first recording layer 64.
To reproduce information from the digital disk set forth above, focus control is conducted in which the position of an object lens 66 is controlled to focus optical beams coming through the object lens 66 onto a specified recording layer. For the focusing control, it has been well known to use optical detectors that are symmetrically disposed with respect to an optical axis to separately detect the quantities of light of the optical beams that return through the object lens 66. A signal indicating a difference between the quantities of light of the optical beams incident upon the two optical detectors is processed and output as, for example, a focus error signal as shown in FIG. 12. Based on this signal, the position of the object lens is adjusted.
More specifically, when the object lens is first moved to shift the focus thereof from an initial position, namely, point A, at the rear of a recording layer 67, toward the second recording layer 62 on the front side as shown, for example, in FIG. 11, no focus error signal will be detected by the optical detectors if there is a large defocus. When the object lens has moves to a predetermined distance or shorter, the positive quantity of light received on one optical detector increases with a resultant gradual increase in amplitude given as an output. In an example shown in FIG. 12, the amplitude reaches its peak at a point of 8 .mu.m (-8 .mu.m) at the rear of the recording layer. Thereafter, as the focus gradually approaches the recording layer, the level of the signal indicating the difference between the two optical detectors decreases because the quantity of light received by the other detector of the two detectors increases. The moment the focus reaches the recording layer, the quantities of light received by the two detectors become equal, and the amplitude representing the differential signal accordingly reaches zero, producing a zero-crossing state.
After that, as the focus moves from the second recording layer 62 toward the front side, the negative quantity of light received by the other detector increases. The increase reaches its peak also at a point of 8 .mu.m (+8 .mu.m) on the front side of the recording layer. Thereafter, as the focus further approaches the front side, the quantity of received light decreases until the detectors finally can no longer receive the beam, causing the focus error signal to become zero. The intermediate layer 63 is normally approximately 40 .mu.m thick, so that it is hardly influenced by the first recording layer 64. Feedback control is carried out in accordance with such a focus error signal so as to carry out the focus control by moving the object lens to a position where the value of the signal reaches the zero-crossing. The focus control for the first recording layer 64 is conducted in the same manner as that for the second recording layer 62.
The focus control based on the focus error signal set forth above in a digital disk such as a DVD provided with a plurality of recording layers is conducted as follows. In the disk shown in FIG. 11, for instance, of the two recording layers 67 of the disk, the focus is first moved to an initial position, namely, point A, farther from the second recording layer 62 facing the substrate 61. Then, the focus is gradually moved toward the front as observed from the object lens. A recording layer is identified by recognizing a first focus error signal obtained as a signal from the second recording layer 62 and a signal thereafter as a signal from the first recording layer 64, and the focus control on the identified recording layer can be implemented. Conversely, the focus may be first moved to an initial position, namely, point B on the front side from the first recording layer 64 located farther from the substrate 61, of the two layers of the recording layer 67. Then, the focus is gradually moved toward the rear as observed from the object lens. A recording layer is identified by recognizing a focus error signal which is obtained first as a signal from the first recording layer 64 and a signal thereafter as a signal from the second recording layer 62, and the focus control on the identified recording layer can be implemented.
Reproduction from a digital disk having two recording layers (hereinafter referred to as "layers") as described above is performed as follows. For instance, as illustrated in FIG. 13, the focus is first moved to point A, namely, the initial position at the rear of the disk, and if an external reproduction instruction indicates reproduction from the second layer 62 formed on the surface of the substrate 61, the focus control is carried out in a position where the first focus error signal was output. After that, if an instruction indicating reproduction from the first layer 64 located farther from the substrate 61 is received, then drive control of "layer jump" to the first layer 64 is carried out and control for focusing on the first layer 64 is conducted.
However, when performing the layer jump, there are cases wherein a scratch on the layer prevents a proper focus error signal from being obtained. There are also cases wherein no focus error signal is obtained if the drive control of the layer jump is excessively quick with resultant overshoot of a lens and a range of a focus error signal output shown in FIG. 12 in the jump layer is exceeded. In addition to such a layer jump failure, there is a defocus problem in which focus control fails to be properly carried out on the layer to jump onto, and focusing cannot be achieved even after the control is continued. If these failures take place, the focus is returned to point A, which is the initial position, in order to carry out control for focusing on the first layer 64 as shown in FIG. 13. This applies also to a layer jump to the second layer 62 when the initial position of the focus is set at point B on the front side of the substrate 61. FIG. 13 is a drawing for illustrating a layer jump; an optical axis of an optical beam does not laterally move in actual use.
Always returning to the initial position before re-focusing on a predetermined layer after an improper layer jump problem, such as a layer jump failure or defocus, takes place has posed the shortcomings described below. As in the example, for instance, when the initial position of the focus is point A at the rear side of the substrate and if a layer jump target is the first layer 64 located farther from the substrate, then the jump takes more time. Moreover, it is necessary to perform signal processing for canceling a focus error signal that is input when the second layer 62 is first passed, then to verify passing of the second layer 62 and thereafter to implement signal processing to input the focus error signal from the first layer 64. This has led to disadvantages of complicated processing circuitry, more processing time required, and a failure to achieve quicker layer jumps. The same shortcomings are observed also in the case wherein the initial position of the focus is set at point B at the front side of the substrate 61. This also applies to the case where more than two layers are formed as recording layers.